Solder ball with chemically and mechanically enhanced surface properties

ABSTRACT

A metal alloy solder ball comprising a first metal and a second metal, the first metal having a sputtering yield greater than the second metal. The solder ball comprises a bulk portion having a bulk ratio of the first metal to the second metal, an outer surface, and a surface gradient having a depth and a gradient ratio of the first metal to the second metal that is less than the bulk ratio. The gradient ratio increases along the surface gradient depth from a minimum at the outer surface. The solder ball may be formed by the process of exposing the ball to energized ions of a sputtering gas for an effective amount of time to form the surface gradient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of allowed U.S. patentapplication Ser. No. 09/382,221, filed on Aug. 24, 1999, now granted asU.S. Pat. No. 6,210,547, which is (a divisional application of Ser. No.09/113,445 filed on Jul. 10, 1998, now granted as U.S. Pat. No.6,056,831), and which is a divisional of U.S. patent application Ser.No. 09/302,740, filed on Apr. 30, 1999, now granted as U.S. Pat. No.6,250,540.

TECHNICAL FIELD

The present invention relates generally to solder balls used inmicroelectronics and, more specifically, to a solder ball having surfaceproperties enhanced by an ion-bombardment-induced sputtering process.

BACKGROUND OF THE INVENTION

In the manufacture of microelectronic devices, various microelectroniccomponents are electrically joined together into circuits using metalalloy solders. Such solders are generally alloys of tin (Sn) and lead(Pb), but can comprise other metal components. The metal alloy solder isgenerally optimized for bulk properties that control the thermalbehavior of the solder. For example, the melting temperature of Pb/Snsolder is controlled by adjusting the relative amounts of tin and leadin the solder. The minimum melting temperature of a Pb/Sn alloy isachieved when the ratio by weight of Pb to Sn in the solder is 37:63.This is called the eutectic composition.

A solder that is homogenous across its mass and optimized for bulkproperties does not take into account the desirable surface propertiesof the solder. Therefore, solder compositions that are optimized forbulk behavior are not necessarily optimized in terms of surfaceproperties. Such surface properties control wetting to metal surfaces,reactivity with organic materials such as fluxes, and the melting pointof the surface layer. Such surface properties depend upon both thechemical composition of the solder and the physical characteristics ofthe surface. For instance, a rough surface may have more surface areafor holding flux than a smooth surface.

For some applications, it is desirable to use bulk solder alloys atother than the eutectic composition. For example, Controlled CollapseChip Connection (C4) solder balls, which are used to join IntegratedCircuit (IC) chips to chip carriers, are typically Pb-rich in the rangeof 90% to 97% Pb. Because Pb-enriched solders have higher meltingtemperatures than solders closer to the eutectic composition, the solderjoint formed by Pb-enriched solders between the chip and the chipcarrier maintains its shape throughout subsequent assembly of the chipcarrier (with chip attached) to another electrical component such as aprinted circuit board. The Pb-enriched solders also require highertemperatures, however, to fuse them to metal surfaces such as copperpads on chip carriers. Thus, organic laminate chip carriers havingcopper joining pads may be exposed to undesirable high temperaturesduring joining processes.

A tradeoff results: without further treatment, the benefit of using aPb-enriched metal alloy solder that maintains its shape at hightemperatures is offset by the need for a high fusing temperature. Suchhigh fusing temperatures can be avoided by applying a eutectic solder tothe joining pads on the organic laminate chip carrier. To ensure properfusing at every joint, however, the height of the applied eutecticsolder layer must be controlled within very narrow limits, a requirementthat is difficult to satisfy.

Another modification that retains high temperature stability whileproviding low temperature fusing comprises placement of an additionallayer or cap of tin or other low melting point metal on the surface ofthe Pb-enriched solder, as described in U.S. Pat. Nos. 5,634,268 and5,729,896 and assigned to the common assignee of the present invention.Such a cap enables the bulk solder to retain its desirablehigh-temperature resistance while providing a surface layer capable offusing at low temperatures.

The present invention proposes a solder ball having an integral soldersurface layer capable of fusing at low temperatures while maintainingthe bulk solder properties.

SUMMARY OF THE INVENTION

The present invention provides a metal alloy solder ball comprising afirst metal and a second metal, such as lead and tin respectively, andhaving a bulk ratio of the first metal to the second metal, the solderball having enhanced surface properties. The process for enhancing thesurface properties comprises exposing the solder ball to energized ionsof a sputtering gas for an effective amount of time to form a surfacegradient having a depth and a gradient ratio of the first metal to thesecond metal that is less than the bulk ratio. The gradient ratioincreases along the surface gradient depth from a minimum at the outersurface.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1A is an atomic-level, side view cross-section of a portion of amass of solder, showing a homogenous bulk mixture throughout;

FIG. 1B is an atomic-level, side view cross-section of the same portionof the solder mass shown in FIG. 1A, showing the impact of surface layermodification via ion bombardment;

FIG. 2 is a side view illustrating a typical apparatus for carrying outthe present invention inside a vacuum chamber, showing substrates heldupside-down by a holder in the path of an ion beam directed by an iongun;

FIG. 3 is a side view illustrating an alternate ion gun embodiment ofthe present invention being used on a substrate shown right-side-up incross-section;

FIG. 4 is a side view cross-sectional illustration depicting Snconcentration along the depth of a solder ball; and

FIG. 5 is a depth profile graph showing sputter time versus atomicconcentration for a solder ball of the present invention, as measured byX-Ray Photoelectron Spectroscopy.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawing, in which like reference numbers refer tolike elements throughout, FIGS. 1A and 1B show an atomic-levelillustration of a portion of a metal alloy solder ball 32. Thisparticular metal alloy solder ball is shown in FIG. 1A having ahomogenous composition of Sn atoms 40 and Pb atoms 42. The Pb atomscomprise roughly 90% of the weight of the solder (a bulk ratio ofapproximately 1:5.16 Sn atoms to Pb atoms).

The process of the present invention comprises altering the surfaceproperties of the metal alloy solder ball by sputtering. The processcomprises exposing the solder for a predetermined time period sufficientto energize ions of a sputtering gas and form a surface layer having adesired depth on the solder and having a ratio of tin-to-lead that isless than the bulk ratio. The energized ions may have an energy in therange of about 0.5 to about 10 KeV and the time of exposure may be inthe range of about 30 seconds to about 60 minutes, resulting in asurface layer having a depth ranging from about 1,000 Angstroms to about2 microns.

FIG. 1B shows the result of this process when the solder is hit by ionbeam 30. When an ion 44 of the ion beam 30 hits a metal atom, such as Pbatom 42′, the metal atom can be sputtered out of the solder as shown inFIG. 1B. The percentage of metal atoms actually sputtered from thesolder per contact with an energized ion is referred to as the“sputtering yield.” The sputtering yield for Pb is typically four tofive times greater than the sputtering yield for Sn. Thus, as the ionbeam 30 hits the surface layer 46 of metal alloy solder ball 32, the Pbatoms 42 are preferentially removed.

The resulting mass of solder has a surface layer 46 with a higher ratioof Pb to Sn atoms, while the original homogenous composition remains inthe bulk of the mass of solder below the surface layer. Thus, thesurface layer becomes Sn-enriched, approaching the same ratio as aeutectic alloy (approximately 1:3 Pb atoms to Sn atoms), while theremainder of the mass of metal alloy solder ball 32 is unchanged.

Thus, the process of the present invention changes the surface layer 46of metal alloy solder ball 32 in two ways. First, the ratio of Pb to Snatoms is changed (a chemical change) in surface layer 46. Second, theroughness of the surface layer is also changed (a physical change) fromthe smooth surface shown in FIG. 1A to the rough surface shown in FIG.1B. The change in chemical composition to a higher Sn concentrationpromotes a lower melting temperature. Thus, a lower temperature isrequired to melt the surface layer 46 for fusing. The bulk solderfeature still maintains its shape, however, during high temperatureprocessing such as joining of chips to chip carriers and chip carriersto printed circuit boards. The chemical change also promotes betterresponsiveness to flux because Sn has a higher reactivity to flux thanPb, while the physical change to a rougher surface promotes the abilityto hold more flux. The roughened surface may also be beneficial forcertain mechanical interconnection schemes that rely on surfaceroughness to frictionally hold components together.

The process of the present invention is also reworkable. The soldersurface layer 46 can be redistributed into the bulk by a meltingoperation, regenerating a homogenous mixture with a smooth surface, ifrequired.

Referring now to FIG. 2, there is shown a sputtering apparatus 10typical of an apparatus that may be used with the process of the presentinvention to modify the surface properties of a metal alloy solder byion bombardment. Vacuum chamber 12 contains a holder 14, for holdingsubstrates 16, and an ion gun 18. As shown in FIG. 2, ion gun 18 directsa stationary ion beam 30 at substrates 16 mounted upside-down on holder14, while the holder moves the substrates in and out of the beam. Holder14 as shown in FIG. 2 is a planetary motion device such as that commonlyused in chemical vapor deposition (CVD) systems and well known in theart.

A vacuum pump 20 is provided to evacuate vacuum chamber 12 via vacuumhose 22. A vacuum gauge 24 may indicate the amount of vacuum inside thechamber. Sputtering gas is supplied to the ion gun via gas tubing 26.

In operation of apparatus 10, vacuum pump 20 evacuates the vacuumchamber 12 and sputtering gas is supplied to ion gun 18 through gastubing 26. A plasma (not shown) is generated within the ion gun, inwhich a voltage grid (not shown) extracts ions from the plasma andaccelerates them out of the gun in a directed ion beam 30 that bombardssubstrates 16 with ions. The amount of exposure time necessary to modifythe solder surface depends on the size and strength of the beam, thedesired depth of modification, and the surface area of the sample to betreated, and can vary from a fraction of a minute to approximately anhour.

Referring now to FIG. 3, there is shown a close-up, cross-sectional sideview of a right-side-up substrate 16 and an alternate type of ion gun18′. Substrate 16 typically is made of a polyamide-, polyimide-,polyester-, or polyethylene-based material or some other organiccomposition, but may also be made of an inorganic material such as SiO₂,Si₃N₄, and the like. Substrate 16 has a plurality of metal joining pads31 on its surface, preferably made of copper, on each of which isdeposited a mass of functional solder 32 or process control solder 32′.Process control solder 32′ is a test feature deposited on the substratesolely for the purpose of process control. Solder 32′ has an areasufficiently large and flat enough to be analyzed by Auger ElectronSpectrometry (AES), X-ray Photoelectron Spectroscopy (XPS), or otherconventional surface analytical techniques. Such analysis is used toevaluate the solder surface to assess the results of the ion bombardmentprocess.

A mask 34 is placed over substrate 16 to shield from ion beam 30′ thoseportions of the substrate not intended for bombardment with ions. Theuse of a mask is especially important when exposure to ion beamirradiation is detrimental to the performance of the materialsurrounding the solder-coated areas. A mask may be omitted if it isdesired to simultaneously modify the substrate surface by ionbombardment while the solder surface is modified. When no substratemodification is desired, the mask may cover all of the substrate exceptthe solder masses. As shown in FIG. 3, mask 34 exposes only soldermasses 32 and 32′ through apertures 33. Alternatively, the mask maycover only the portions of the substrate or attached features sensitiveto and not intended for bombardment.

Ion gun 18′ shown in FIG. 3 has a head 19 capable of directing arastered ion beam that can be directed across stationary substrate 16. Arastered beam essentially moves in a line from side-to-side of thesubstrate starting from one edge, and indexing the width of the beam ina direction perpendicular to the side-to-side motion between eachside-to-side pass, so that at the completion of rastering, the beam hascovered every portion of the substrate. The ion gun 18′ shown in FIG. 3is also placed within a vacuum chamber (not shown) and supplied withinert gas (not shown) as was the embodiment shown in FIG. 2.

An ion gun apparatus producing a rastered ion beam may also be used inconjunction with a moving sample such as on a conveyor, where aplurality of holders in series each brings one substrate into a positionwhere it is contacted by the ion beam. In such a configuration, the ionbeam may merely travel from side-to-side in one direction, while theconveyor provides the indexing motion of the substrates in theperpendicular direction to provide total coverage of the substrate.

Other ion generators may be used instead of ion guns. Ions can beproduced in an electrical discharge or plasma, such as an argon plasma.The naturally occurring space charge sheath that develops at allsurfaces in contact with the plasma may provide the necessaryacceleration of these ions toward the solder surface. Because ionshaving the highest kinetic energy will generally strike the poweredelectrode of the plasma generator, the surface to be bombarded maybenefit from being placed on the powered electrode. Placement elsewherein the plasma may be sufficient, however, to produce the desired effect.Thus, a stationary or moving substrate with its associated mask maymerely be placed in a plasma for surface preparation of the solder.Nevertheless, to ensure that the ions attain sufficient energy forsputtering metal atoms such as Pb, an ion gun for acceleration andpropagation of a beam of ions is preferred.

For small substrates, the entire area to be treated can be irradiated byions with a stationary substrate and stationary ion beam. For largersubstrates, however, it may be necessary to either move the ionirradiation over the area to be treated, such as by rastering a beam ofions as shown in FIG. 3, or to move the substrates through a stationaryzone of treatment such as shown in FIG. 2.

The composition of the mask 34 is important. To extend the useful lifeof the mask and to avoid generation of potential contamination of thesubstrate 16 by sputtered fragments from the mask, mask erosion must beavoided. This is best accomplished by use of a mask material having asputtering yield far lower than that of the element to be preferentiallysputtered from the alloy being modified. For example, for enrichment ofSn on the surface of Pb—Sn alloys, molybdenum, titanium, and zirconiumare desirable mask materials. For ease of mask fabrication, molybdenumis preferred.

The vacuum chamber 12 is fabricated from suitable materials ofconstruction such as aluminum, stainless steel, quartz, chemical andtemperature resistant glass, or the like.

Noble gases such as argon are typically used for sputteringapplications, and thus are also suitable for Sn enrichment of soldersurfaces according to this invention. If simultaneous Sn enrichment andsubstrate surface chemical treatment are desired, however, high energyions generated from a reactive gas, such as oxygen, can be used. Whenoxygen gas is incorporated into the sputtering gas mixture, asignificant amount of substrate modification (roughening orincorporation of chemical functional groups of organic material) can beachieved simultaneously with the solder surface modification. Suchroughening may be used to improve adhesion between an organic substrate,such as a laminate, and any encapsulating layer, such as a polymer,deposited on the substrate to reinforce the solder joints and to reducefatigue damage.

Substrate surface roughening simultaneous with solder surfacepreparation provides an advantage over processes using low-melting pointcapping of C4 solder. To obtain a roughened, low-melt surface, thecapping technologies require both a step to provide the cap and a stepto plasma-roughen the substrate, whereas the process of the presentinvention requires only the plasma step that can both roughen thesubstrate and prepare the solder surface.

To provide such a reactive plasma, a reactive gas such as oxygen may beintroduced to the sputtering application directly into the ion gun 18 orother plasma generator through gas tubing 26 along with inert gas. In analternate embodiment, a plasma generator external to vacuum chamber 12may generate the reactive gas ions and pipe the energized ions into thechamber through a dedicated line. In yet another alternate embodiment, areactive gas plasma generator may be inside the vacuum chamber 12 withits own gas supply piping to provide reactive gas for generatingreactive ions from the gas.

In addition to use with Pb—Sn alloys, the present invention may also beused with ternary alloys that contain Sn, Pb, and a third component suchas silver (Ag). The process and resulting structure of the presentinvention may also be extended to alloys comprising metals other than Snand Pb.

It should be understood that the term “layer” as used in this documentrelates to regions in the metal alloy solder 32 having certainproperties, such as being Sn-enriched. For convenience, such regions areillustrated in FIGS. 1A and 1B as being well-defined regions. Suchregions are not, however, discrete, homogenous layers. Rather, theregions are layers comprising transitional gradients, such as is shownin FIGS. 4 and 5. For instance, FIG. 4 shows a solder ball 60, mountedon solder pad 48 on substrate 50, having a Sn-enriched surface layer 46comprising a surface gradient with a greater Sn enrichment (minimumPb:Sn ratio) at outer surface 146 than close to bulk solder 32. The term“layer” therefore includes such non-homogenous, surface gradients havinga Pb:Sn ratio that increases along the depth of the surface gradientfrom a minimum at the outer surface. In a preferred embodiment, the bulksolder has approximately a 90:10 Pb:Sn weight ratio and the gradientratio ranges from a minimum of greater than or equal to about 37:63 to amaximum of the bulk ratio. The bulk ratio may be greater than 90:10,such as 97:3, for example.

FIG. 5 illustrates a depth profile of relative atomic concentrations fortin and lead obtained using X-ray Photoelectron Spectroscopy (XPS) for asample of 90:10 Pb:Sn solder exposed to an argon ion beam in accordancewith this invention. The argon ion beam used for enhancing the solderwas operated at an energy level of 3 kV and a current of 25 mA at apressure of 1×10⁻⁸ Torr. XPS can typically sample the surface layer to adepth of about 50 to about 100 Angstroms. FIG. 5 shows the percentconcentration of each element: lead (Pb), tin (Sn), and oxygen (O) as afunction of sputter time. XPS functions by sputtering away the outerlayer of solder while continually measuring the concentration of theouter layer. Thus, the profile of concentration versus sputter timeshown in FIG. 5 is essentially a profile of concentration versus depth.

As shown in FIG. 5, the atomic ratio of Pb to Sn is much less at 0minutes than at 15 minutes. The approximately 38% Pb, 28% Sn, 34% Oatomic concentration at 0 (zero) minutes corresponds to approximately a70:30 Pb:Sn weight ratio on the outer surface of the solder ball,whereas the 70% Pb, 21% Sn, 1% O atomic concentration at 15 minutescorresponds to approximately an 85:15 Pb:Sn weight ratio in the bulksolder. The preferential sputtering rate of Pb during the XPSmeasurement process skews the measured Pb:Sn ratio lower than the actual90:10 weight ratio in the bulk, but the 1% (nearly zero) atomicconcentration of oxygen at the 15 minute mark indicates that the XPS hasreached the bulk solder. FIG. 5 thus depicts the surface gradient of thepresent invention, showing how the Pb:Sn ratio changes gradually betweena minimum at the 0 minute mark to the bulk ratio at approximately the 15minute mark.

It is further noted that the masses of solder comprise “solder balls”(also sometimes called “solder bumps”) as are commonly known in the art.Solder balls are rounded masses of solder, such as result from a reflowstep during which a mass of solder tends to ball-up into a roundedsurface. Thus, the process of this invention generally is conductedafter a reflow step, and typically results in a reflowed solder ballhaving the surface gradient described above.

EXAMPLES

The following examples are included to more clearly demonstrate theoverall nature of the invention. These examples are exemplary, notrestrictive, of the invention.

Example 1

A Pb/Sn metal alloy solder sample having a composition of 40% Pb byweight was placed in a vacuum chamber evacuated to 1×10⁻⁷ to 1×10⁻⁸Torr. Using an ion gun and argon as a sputtering gas, an ion beam of 4KeV argon ions having a raster size of 3×3 mm was directed at thesolder. The solder was sequentially sampled over time starting at afraction of a minute to an hour of exposure to determine the impact ofthe ion bombardment over time. Sampling was performed by XPS. Ionbombardment was shown to be capable of modifying the surface layer ofthe 40:60 Pb:Sn alloy to a 23:77 Pb:Sn composition.

Example 2

A eutectic Pb/Sn solder sample was placed in a chamber at 0.13 Torr. Aplasma was generated having a total applied power of 6,500-7,000 Wattsat 40 KHz, resulting in 0.1 Watts/cm² of powered electrode area, using agas having a composition of 93% argon and 7% oxygen. The sample wassubjected to the plasma treatment for one hour. A significant increasein roughness of the solder balls was recognized in SEM micrographs, andthe solder composition changed from the original level of 35:65 Pb:Sn byweight to 15:85 Pb:Sn by weight. The testing showed that, for thisexample, the oxygen content needed to be below 15%, with less than 10%oxygen being preferred. Higher oxygen levels formed thicker oxide layersthat lessened the probability of sputtering. Also, pressures above 0.225Torr tended to lessen the sputtering effect.

Example 3

A Pb/Sn metal alloy solder sample having a composition of 90% Pb byweight was placed in a vacuum chamber evacuated to 0.085 Torr. A plasmawas generated using argon at a flow rate of 14.3 standard cubiccentimeters per minute and a 300 Watt power source with 13.56 MHzfrequency applied at the cathode, resulting in 0.45 Watts/cm² coverageof the sample. The resulting plasma had an average peak-to-peak (V_(pp))energy of 1,240 Volts and a self-bias or DC-offset (V_(dc)) of 200Volts. The sample was treated for 45 minutes with continuous samplingand analysis to determine the degree of surface change. Over the courseof treatment, the Pb/Sn atomic concentration decreased.

Although illustrated and described with reference to certain specificembodiments, the present invention is nevertheless not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the spirit of the invention.

What is claimed:
 1. A metal alloy solder ball having an outer surfaceand comprising a first metal and a second metal, the metal alloy solderball having a bulk ratio of the first metal to the second metal in abulk portion, the first metal having a sputtering yield greater than thesecond metal, the metal alloy solder ball comprising a product producedby a process for altering surface properties of the metal alloy solderball, the process comprising: exposing the metal alloy solder ball toenergized ions of a sputtering gas for an effective amount of time toform a surface gradient having a depth and a gradient ratio of the firstmetal to the second metal that is less than the bulk ratio, the gradientratio increasing gradually along the surface gradient depth from aminimum at the outer surface.
 2. A metal alloy solder ball comprising: afirst metal and a second metal, the first metal having a sputteringyield greater than the second metal; a bulk portion having a bulk ratioof the first metal to the second metal; and a surface gradient betweenan outer surface of the solder ball and said bulk portion, the surfacegradient having a depth and a gradient ratio of the first metal to thesecond metal that is less than the bulk ratio, the gradient ratioincreasing gradually along with surface gradient depth from a minimum atthe outer surface.
 3. The metal alloy solder ball of claim 2 wherein theouter surface is rough as compared to an outer surface of an otherwiseidentical solder ball that has not undergone a roughening step.
 4. Themetal alloy solder ball of claim 2 wherein the solder ball comprises areflowed solder ball on which the surface gradient has been formed aftera reflow process.
 5. The metal alloy solder ball of claim 2 wherein thesecond metal is tin.
 6. The metal alloy solder ball of claim 5 whereinthe first metal is lead.
 7. The metal alloy solder ball of claim 6wherein the second metal is tin.
 8. The metal alloy solder ball of claim7 wherein the gradient ratio has a minimum of greater than or equal toabout 37:63 lead:tin by weight.
 9. The metal alloy solder ball of claim8 wherein the bulk ratio is greater than or equal to about 90:10lead:tin by weight.
 10. A metal alloy solder ball comprising: lead andtin, the lead having a sputtering yield greater than the tin; a bulkportion having a bulk ratio greater than or equal to about 90:10lead:tin by weight; a rough outer surface as compared to an outersurface of an otherwise identical solder ball that has not undergone aroughening step; and a surface gradient between the outer surface of thesolder ball and the bulk portion, the surface gradient having a depthand a gradient ratio of the lead to the tin that is less than the bulkratio, the gradient ratio increasing gradually along with surfacegradient depth from a minimum at the outer surface.
 11. The metal alloysolder ball of claim 10 wherein the solder ball comprises a reflowedsolder ball on which the surface gradient has been formed after a reflowprocess.
 12. The metal alloy solder ball of claim 10 wherein thegradient ratio has a minimum of greater than or equal to about 37:63lead:tin by weight.